Study On Low-reservoir-damage Fracturing Fluid For Continental Shale Of Chang 7 Member

The large-scale production of shale gas in North America has caused the attention of other counties all over the world of shale gas resources.Until now,a big success has been made in America with mature technology of shale gas exploration and development.However,shale gas development in China is still in the initial stage and restricted by several limitations,including lacking experience,immature technology,and so on.Fortunately,a breakthrough of continental sedimentary shale exploration gas has been achieved in the Ordos Basin.Series of achievements were made through the pilot evaluation and exploration test in the Mesozoic Triassic Yanchang Fromation.Compared to the marine shale gas reservoirs in North America and South China,the reservoirs in Ordos Basin shown different mineral composition and physical properties because of their lake sedimentary environment,which can be characterized as continental shale gas reservoirs.This is why the technology and experience of marine shale gas development cannot be directly used for the development of continental shale gas.It is advantageous to extend the shale gas in the 7th section of Yanchang Formation because of the higher organic matter conversion degree,and it is urgent to break the bottleneck of fracturing fluid technology in this area.Based on the practical reality of lower production in continental shale gas reservoirs,we extend the research on the low-damage fracturing fluid system suitable for Chang 7 member of Yanchang shale-gas reservoirs in this paper.Comprehensive tests were conducted in laboratory,and several technologies and methods about fracturing fluid were summarized to lower the reservoir damage.This paper can be divided into seven sections,and the main contents of each section are summarized as follows:The first chapter: Based on the summarization of research background,goals and significance,the 7th section of Yanchang Formation reservoir was chose as the research object.In order to explore the influencing factors of target blocks production for scientific study,the whole research can be characterized as five key points,including reservoir characterization,fracture making,sand carrying,gel breaking and low reservoir damage.The second chapter: Based on the data collection and laboratory experiment,the geological characteristics of the target reservoirs were analyzed,including reservoir effective thickness,natural pore development,temperature and pressure system and the characteristics of the rock mechanics.Fundamental reasons of production difference between continental shale gas and marine shale gas were found out by comparison of their shale samples.The third chapter: After identifying the geological characteristics of the continental and marine reservoirs,the differences of reservoir damage caused by fracturing were studied.The damage caused by water sensitivity and polymer invasion were studied respectively,including shale swelling,dispersion,water lock and salinity sensitivity as a reason of shale composition difference,and the surface damage caused by polymer fracture fluid system.Finally,fracture fluid system suitable for the target block with special demands was put forward.The fourth chapter: The drag reduction mechanism of slick water system was analyzed and tested,and the factors of drag reduction rate were determined.A device for drag reduction rate was designed and basic tests were carried out.Single agents were optimized and then combined for slick water system formulation design.Comprehensive tests of the formulation were carried out,especially for the performances of acid and temperature resistance.The fifth chapter: Advantages and disadvantages of biological enzyme degradation were compared,and the gel breaking mechanism was studied.Influences of addition and temperature on enzyme effect were investigated.And based on the test results,viscosifiers,crosslinking agents,gel breaking agents and bactericides were optimized.And effects of temperature and acidity on the agents mentioned above were studied emphatically.The sixth chapter: An evaluation device was designed in lab to analyze the rules of commercial proppant sedimentation and migration,and the factors influencing the proppant sedimentation and migration were summarized.Based on the comparison of modified polypropylene and commercial proppant,whether the material can be used for the target block has been studied.The seventh chapter: The main conclusions and innovations of this paper were summarized,and the shortcomings and the further research direction were put forward.Based on the research of the problems mentioned,the conclusions are as follows:(1)Water sensitive damage will occur because of the low brittle mineral content and high clay mineral content in 7th section of Yanchang Formation continental shale gas reservoirs.High pressure filter will not be avoided for the weak structure matrix and the development of structural fracture and bedding fracture.Fracture net cannot be formed immediately as a reason of higher poisson's ratio and smaller elastic modulus.(2)The results show that the surface of the shale with water intrusion has a rapid expansion of the surface,Nano-level cracks will closequickly,cementability of clay minerals are strong,and the liquid-solid two-phase mixing is deep into the fracture and blockthe channel,resulting in the nanopore shrinkage and the porosity decrease.There are obvious differences in the core rate of different mineralization solution immerse reservoirs,and the low-seepage trend with the increase of the small range.With increasing mineralization solutionwill infiltrate the shale rate of regression,and this closed-loop relationship indicates that the salt solution intrusewill exsit an extreme value of the solution intrusion core rate,which is the optimal amount of the slowest and fastest permeation rate.Plant gums such as SG are easy to cover the core surface of the reservoir,the linear molecular polymer PAM mostly fissure invasion damage.(3)Cationic surfactants can effectively reduce the surface tension,and the surface tension increases with the number of alkyl increasing.Ammonium chloride is more effective than ammonium bromide to reduce the surface tension when the alkyl number is the same.A better wetting reversal effect can be seen for cationic surfactants,of which the hydrolyzed cation is of great importance to wetting inversion of shale surface compared to anionic surfactant and zwitterionic surfactants.Especially for the double ammonium cationic surfactant,the wetting reversion effect was more obvious.The contribution rate of anti-water lock is higher than 60% when it comes to cationic surfactant,and the anti-water lock effect is not very different with surfactant addition changing.(4)The flow resistance can be significantly reduced with the addition of lubricant and auxiliary drag reducing agent.The anionic PAM exhibits a better drag reduction performance and there is an optimal addition.A good drag reduction effect can be obtained when the molecular weight is 12 million for nonionic PAM.Besides,the drag reduction effect of PAM is affected by formation water quality,drag reducing agents should be optimized according to fracture technology because they will deposit in the acid or alkaline environment.The drag reducing agents is sensitive to temperature,and there exists optimal temperature for drag reducing effect.When the temperature exceeds 70?,the antioxidant should be added.Some of the drag reducing agents can decrease the flow resistance over 80%.(5)The biological enzyme exhibits a good gel breaking effect to the polymers at low temperatures.Gel breaking efficiency increased with the biological enzyme concentration increasing.Most of cellulose-based polymers can be degraded by cellulase which has the optimum reaction temperature,and the gel breaking efficiency will be reduced when the temperature is too high or too low.XC cannot be degraded effectively by normal biological enzyme collected from the market.(6)When the viscosity of the plant gum crosslinking system increased from50 mPa.s to 100 mPa.s,the settlement rate will decrease 42.5% and 75% for fracture proppant of 20 mesh and 40 mesh respectively.The settlement rate of 40 mesh fracture proppant decreaed 70.8% compared to 20 mesh fracture proppant in the base fluid.Blockage will occur once the particle diameter of the proppant is bigger than the 1/3 of seam width.Cavitation damage of proppant will occur in the rapid flow of perforation.(7)The modified polypropylene proppant is not broken at 20 MPa,but a significant plastic deformation can be observed,which prevent the damage of shale surface.The combination of modified polypropylene in small particle size and the ceramsite in large particle size improve the deposition problem when the ceramsite used.